A light output or a light extraction output of a semiconductor light-emitting element may be limited by a surrounding medium such as air or an epoxy resin and a semiconductor material due to the difference between the refraction indices of these materials. For example, if an AlGaInP type semiconductor material for a light-emitting element (refraction index SEMI=3.3) and a resin (refraction index RESIN=1.5) are used, the critical angle is 27 degrees and the reflectance of the boundary therebetween is about 15%, meaning that the extractable light to the outside may be limited to about 4.5%.
Incidentally, a semiconductor layer in a semiconductor light-emitting element can be produced on a growth substrate such as a GaAs substrate for an AlGaInP layer and a sapphire substrate for a GaN layer. In order to achieve a high efficiency, deterioration of light output due to the absorption or confinement of light emitted from the active layer to a growth substrate should be prevented.
In particular, a semiconductor light-emitting element formed from an AlGaInP type material can be produced by an MOCVD method on a GaAs substrate that can be lattice-matched to the AlGaInP type material. However, in a semiconductor light-emitting element formed from the AlGaInP type material, the bandgap of the active layer is larger than the bandgap of the GaAs substrate. Thus, part of light emitted and directed to the light extraction surface can be extracted, but light directed to the GaAs substrate may be absorbed by the GaAs substrate, thereby deteriorating the efficiency.
Conventionally, an MOCVD method can be used to produce a semiconductor element on a GaAs growth substrate. In another known art, such a GaAs growth substrate is removed and then a different substrate is provided via a metal to form a semiconductor light-emitting element (referred to as an MB structure). For example, Japanese Patent Application Laid-Open No. 2007-227895 discloses a semiconductor light-emitting element having such a structure. Such a semiconductor light-emitting element is provided with a reflection minor to the opposite side to the light extraction surface of the element. The light that has been absorbed by the growth substrate in a conventional case can be reflected to be redirected to the outside, thereby improving the light extraction output. Such a semiconductor light-emitting device can have a semiconductor layer of which surface on the light extraction surface side can be roughened to a random shape, arrangement, size, and the like. The roughening intends to change the propagation direction of light within the semiconductor and extract the light component at angles over the critical angle, thereby improving the light output. In particular, the semiconductor light-emitting element with an MB structure may cause the light to be multiply reflected within the semiconductor, and accordingly, the roughening can largely improve the light output.
Further known is a semiconductor light-emitting element in which the electrode on the light extraction surface side is shifted so as not to overlap with the electrode on the opposite side thereto when viewed from above (so called a counter electrode structure). Such a semiconductor light-emitting element with the counter electrode structure can achieve uniform current diffusion with less electrode coverage, thereby improving light output (see Japanese Patent Application Laid-Open No. 2010-192709).
In the semiconductor light-emitting element with such an MB structure as shown in Japanese Patent Application Laid-Open No. 2007-227895, the light extraction surface is randomly roughened, thereby improving the light output of the semiconductor light-emitting element. However, when roughening of the light extraction surface is simply performed, a plurality of projections are formed on the roughened surface. The tips of the plurality of projections are directed in random directions, and therefore, projections can be formed so that the tips thereof are directed in the same direction as a direction in which current flows through the element (current path). In general, if a semiconductor light-emitting element includes an MB structure having a reflection minor, the semiconductor light-emitting stacked body is composed only of layers produced by an MOCVD method. Since the thickness of the layer is very thin (10 μm or thinner) when compared with the distance between the electrode on the light extraction surface side and the electrode on the reflection surface side, the current path in an in-plane direction is longer than that in a perpendicular direction (in the thickness direction of the layers). Accordingly, if projections are formed so that the tips thereof are parallel to the current path in the in-plane direction, the electric field may concentrate on the tips of the projections, thereby lowering the electrostatic withstanding voltage.
In addition, the shorter the distance between the surface electrode piece and the projections on the roughened surface directed in the same direction as the current path is, the larger the current density near the projections becomes. Thus, the electric field may concentrate more and the electrostatic withstanding voltage may be lowered to a greater extent. The larger area of the semiconductor layer on the light extraction surface side to be roughened (ratio of the roughened surface) can improve the light output, but this configuration may shorten the distance between the projections and the surface electrode piece. Accordingly, it is difficult to simultaneously satisfy the light output and the improvement in the electrostatic withstanding voltage in view of the ratio of the roughened surface, which is a remaining problem of the conventional semiconductor light-emitting element with an MB structure that is to be solved.